The present invention relates to electronic control systems for the automatic regulation of electrical power supplied to various types of process control means in response to the variation of a sensed temperature from a preselected set-point value. The system is disclosed herein in connection with the automatic control of fluid temperature and fluid level in "hot melt" liquid adhesive applicator apparatus.
In the regulation of various process control parameters such as temperature, pressure, fluid level and the like, it is common to employ thermosensitive elements such as thermistors, bimetallic switches or hot bulbs at a number of locations to generate signals which are used to control the application of electrical power to various control means in the system. For example, control means such as heating elements may be used to supply thermal energy to fluid materials to maintain the temperature thereof within preset limits. Typically, the output signal of the temperature sensor is electronically processed and compared with another signal representing a preset or "set-point"" temperature level corresponding to the desired temperature at the monitored location. The difference between the sensed and set-point signals is used to control the application of electrical power to the heating element.
In the case of control means for maintaining the level of heated fluid at a particular position, the temperature sensor may be disposed above the surface of such a fluid contained within a reservoir. When the level of the heated fluid rises it contacts the sensor which generates a signal indicating an increased temperature at the control location. The sensor signal may then be used to control power switching circuitry to close a solenoid actuated fluid valve controlling the flow to the monitored control location.
In the area of the control of fluid temperature in "hot melt" liquid adhesive applicator apparatus, the temperature sensor commonly takes the form of a "hot bulb" or thermistor which produces a signal representative of adhesive temperature. This signal is electrically compared with a fixed signal representing the manufacturer's recommended temperature for the adhesive being used to produce a difference signal that controls a power supply operating heating elements disposed throughout the system.
Existing systems of this nature are generally quite inflexible and any individual system has only a limited power variation capability. In addition, such systems are quite bulky. Moreover, only limited accuracy of control has been possible with such systems. In contrast, the present invention provides a compact, high power, multipoint control system characterized by a high degree of accuracy, electromagnetic and electro-optical isolation, automatic cold junction compensation, thermocouple break protection, high/low visual alarms, and additional other highly advantageous features which will be more readily apparent after reading the following description with reference to the accompanying drawings.